This invention relates to an adaptive equalizer for reducing intersymbol interference in digital communication apparatus that receives differentially encoded data.
Differentially encoded binary phase-shift keying (BPSK) and quadrature phase-shift keying (QPSK) are widely used in cellular radio systems for communication between a base station and mobile stations such as automobile-mounted telephones. The mobile receiver generally includes an equalizer: a digital filter that compensates for transmitter filtering and channel conditions.
Cellular communication is performed on a time-division multiple-access basis and takes place in a series of short bursts. A burst commences with the transmission of fixed training data to enable the equalizer to adapt its filter coefficients, then continues with user data, during the reception of which the equalizer further adapts its filter coefficients to track channel variations.
The equalizer can adapt to gradual channel variations, but cannot follow abrupt changes. When such abrupt changes occur, the equalizer may lose track of the channel condition. If the equalizer wanders too far from the correct channel condition, received data values will be in error.
One way to reduce such errors is to perform a midburst correction using known data, such as a color code occurring at a fixed intermediate location in the user data. Due to the phase ambiguity of differential encoding, however, the receiver cannot tell which of the possible encodings of the known data to employ as desired data for the midburst correction. For this reason, existing adaptive equalizers have been unable to take advantage of known data occurring midway through a burst.